1. Field of the Invention
The present invention relates to vehicular Anti-lock Brake Systems ("ABS") and more particularly, to the control system/method for determining when, and in what manner, braking pressure should be reapplied to pressurized fluid actuated vehicular brakes which have been released in response to a sensed actual or incipient wheel-lock condition.
2. Description of the Prior Art
Anti-lock brake systems for vehicular fluid pressure actuated brakes are well known in the prior art. Briefly, as is well known, to provide a desirable maximized combination of vehicle retardation and lateral and transverse vehicle stability, it is important to prevent and/or quickly terminate occurrences of wheel-lock up. In particular, to provide a maximized combination of vehicle retardation (i.e. braking) and vehicle stability, it is advantageous to maintain the slip rate at about twenty to forty percent (20% to 40%). Prior art vehicular ABS, and ABH valves therefor, to accomplish the above may be appreciated by reference to the following U.S. Patents Nos., the disclosures of which are incorporated by reference: 3,604,760, 3,663,070; 3,709,566; 3,747,989; 3,758,167; 3,880,474; 3,881,779; 3,917,358; 3,929,383; 4,094,556; 4,189,192; 4,511,971; 4,585,280; 4,679,866 and 4,762,375.
The prior art ABS typically utilized an ABS valve having an inlet port and a utilization port connected in series between the brake treadle valve controlled by the operator brake pedal and the fluid operated brakes. The ABS valves also had an exhaust port and one or more controllable valve elements, usually solenoid controlled valves controlled by a microprocessor based ABS controller, to provide a fast fill, slow fill, exhaust (release) and often a hold function. The ABS valves often were operated by pulse width modulation techniques or the like to achieve the slow fill and/or hold functions.
In operation, the inlet to the ABS valve received pressurized fluid at a pressure set by the operator's position of the brake pedal. Initially, the ABS valve remained in a fully or substantially fully open (fast fill) position, with the exhaust port closed until actual or incipient wheel-lock was sensed. The ABS valve then assumed the release (exhaust condition) to rapidly release the brakes until conditions requiring a hold or reapply operation were sensed (usually an indication that the wheels were rolling up to vehicle speed), and then the brakes were reapplied as the ABS valve reassumed a fast fill condition until a condition, such as sensing a pressure slightly less than last pressure at which a lock-up condition was sensed, as achieved, at which time the brakes were applied with a less rapidly increasing pressure as the ABS valve assumed the slow fill mode.
While the prior art ABS do provide increased vehicle retardation and/or stability, they are not totally satisfactory, as pressure sensors as well as wheel speed sensors were required, and/or the control system/method for determining at what point and at what rate of pressure increase to reapply the brakes was not satisfactory for low friction, medium friction and high friction surface conditions.